A method of the kind described above is disclosed, for example, in U.S. Pat. No. 5,898,103 and incorporated herein by reference. German patent publications 195 02 775 and 195 02 776 disclose a method for checking the tightness by means of a difference flow sensor as well as by means of a difference pressure sensor.
All of these methods permit a tightness check to be made on the tank system. However, with these methods, it is not possible, in addition to the tightness, to detect additional variables, which are specific to the tank, or characteristics of the tank system such as the fill level of a liquid disposed in the tank system.
Thus, for example, to check the fill level, additional float lever transducers are required as shown, for example, in FIG. 3 of the drawings herein. These float lever transducers are however difficult to mount and, furthermore, require a complex evaluation of the fill level transducer signal for a complicated tank geometry because, in this case, mostly a nonlinear relationship is present between the fill height and the fill level.
Furthermore, two float lever transducers are required for tanks having two or more chambers which transducers must be separately evaluated in an especially complicated way.
German patent publication 4,203,099 discloses a method for detecting the tank fill level which, on the one hand, makes possible a check of the tightness of the tank and, on the other hand, permits a determination of the fill level thereof to be made. In this method, the tank is charged with a pressure change and a conclusion is drawn as to the fill level of the tank from the value of a pressure-change gradient variable within a pregiven time span. It is a disadvantage of this gradient method that the fill level, on the one hand, can only be imprecisely detected because a gradient determination is always burdened with an error because of difference formations. On the other hand, the method requires a relatively complex configuration with a plurality of sensors and check devices.